This invention relates to ceramic capacitors and in particular, but not exclusively, to multilayer ceramic capacitors and dielectric compositions for use therein.
A multilayer ceramic capacitor basically comprises a stack consisting of a plurality of dielectric members formed of a ceramic material, with electrodes positioned between the members. The electrodes may be screen-printed onto the ceramic material, in the unfired state thereof, using conductive inks. A stack of screen-printed dielectric members is assembled, pressed together, cut into individual components, if appropriate, and fired until sintering occurs, in order to ensure non-porisity. The internal electrodes may be of rectangular form and cover the whole or part of the area of the adjacent dielectric layers. The internal electrodes in successive layers may be sideways stepped relative to one another or have elongate portions which cross one another, as described in British Application No. 7841677 (Ser. No. 2032689A) (A. Oliver-G. Mills 1-1).
With the conventional employed dielectrics the capacitors must be fired at temperatures of the order of 1200.degree.-1400.degree. C., which means that the internal electrodes must be of a suitable material to withstand such temperatures and that, therefore, expensive noble metals, such as platinum or palladium must be used. However, if the firing temperature can be reduced, by a suitable choice of the dielectric, then internal electrodes with a high silver content (50-100% silver) could be used, thus reducing costs for materials and manufacture. In British Application No. 8120605 (Ser. No. 2107300) (J. M. Wheeler 1) there is disclosed a dielectric composition which can be fired at a temperature between 950.degree. C. and 1100.degree. C. and can thus be used with high silver content internal electrodes. These low firing temperature dielectrics comprise lead magnesium niobate PbMg.sub.1/2 Nb.sub.1/2 O.sub.3 with one or more of the following, namely lead titanate, lead stannate, lead zirconate, and some of these dielectric compositions have dielectric constants in the range 7500-10,000, which makes them particularly suitable for multilayer ceramic capacitors. The conventionally employed ceramic (U.S. coding Z5U) which are compatible with high silver content electrodes usually have dielectric constants lower than 6000. The electronics industry, generally, requires smaller components, and smaller and cheaper capacitors can be obtained by producing dielectrics which are compatible with high silver content electrodes and even higher dielectric constants than the 7500- 10,000 range mentioned above with reference to British Application No. 8120605.
In British Application No. 8317265 (Ser. No. 2126575) (J. M. Wheeler 2x) there is disclosed a dielectric composition comprising lead magnesium niobate PbMg.sub.1/2 Nb.sub.1/2 O.sub.3 non-stoichiometric lead iron niobate and one or more oxide additives, which may be chosen from silica, manganese dioxide, ceric oxide, lanthanum oxide, zinc oxide, alumina, tungsten oxide, nickel oxide, cobalt oxide and cuprous oxide. If, for example, three or more oxide additives are chosen from the first eight of the ten mentioned above, compositions having firing temperatures between 980.degree. C. and 1075.degree. C. may be obtained, the dielectric constants after firing being in the range 10,600 to 16,800, making them particularly suitable for small multilayer ceramic capacitors with high silver content electrodes. Additionally the dielectric composition may contain lead titanate (PbTiO.sub.3).